Radiation therapy for cancer treatment has been in use for decades. Modern radiation therapy systems typically generate high intensity x-rays by bombarding a suitable target with high energy electrons. X-rays are emitted from the target in a generally conical pattern, and are initially confined to a generally rectangular beam by moveable, x-ray blocking “jaws” in the head of the system. Typically, the patient is positioned about 1 meter from the x-ray target, and when the jaws are fully open, they define a square treatment area that is about 40 cm×40 cm at the patient plane. However, in many instances it is important to only irradiate a precisely defined area or volume conforming to a tumor, and the target site must be irradiated from multiple angles. Rarely, however, can the system jaws alone be used implement a suitable treatment plan.
Multileaf collimators (“MLCs”), such as described in the co-assigned U.S. Pat. No. 4,868,843, issued Sep. 19, 1989, to Nunan, (the disclosure of which is incorporated by reference), have been almost universally adopted to facilitate shaping of the radiation beam so that the beam conforms to the site being treated, i.e., the beam conforms to the shape of the tumor from the angle of irradiation. Subsequent to its introduction, the MLC has also been used to perform a technique known as “Intensity Modulated Radiotherapy”(“IMRT”), which allows control over the radiation doses delivered to a specific portions of the site being treated. Specifically, IMRT allows the intensity distribution of the radiation reaching the patient to have almost any arbitrary distribution. IMRT can be implemented by iteratively positioning the leaves of the MLC to provide desired field shapes which collectively deliver the desired dose distribution. This approach is static in the sense that the leaves do not move when the beam is on. Alternatively, in systems sold by the assignee of the present invention, IMRT can be implemented using a “sliding window” approach, in which the leaves of the MLC are moved continuously across the beam when the beam is on. Specifically, by adjusting the speed of leaf motion and separation of the leaves, different portions of the treatment field can be irradiated with different doses of radiation.
Heretofore, treatment planning has proceeded based on the use of a beam that is uniform within the entire treatment area defined by the jaws. However, since the x-ray beam emitted from a target is not uniform, it has been necessary to insert a “flattening filter” in the path of the emitted x-rays to achieve beam uniformity. Specifically, a flattening filter attenuates the higher intensity, central portion of the x-ray beam. A flattening filter generally comprises a solid metallic cone of x-ray absorbing material that is inserted into the path of the x-ray beam, such that the center of the cone is coaxial with the electron beam striking the target. Because a flattening filter substantially attenuates the average intensity of the x-ray beam, it can prolong the time needed to provide the desired dose.
Raw particle beams (e.g., electron beams) used in radiotherapy also vary in intensity over the relatively large area of a modern radiation therapy system and, therefore, a structure, such as a scattering foil is used to provide a flattened output beam. To date, the use of MLCs with particle beams is relatively uncommon due to scattering problems, however it is anticipated that those problems may be addressed, such that the use of MLCs with particle beams will become more common.
Many radiation therapy systems are designed to emit x-rays at different energy levels to provide different tissue penetration capabilities, thereby providing additional treatment flexibility. Thus, for example, systems sold by the assignee of the present invention are capable of delivering x-rays in the range of 4 Mev to 23 Mev. (It is noted that the x-ray energy is generally referred to in terms of the energy of the electrons which strike the target. Thus, 6 MeV x-rays refers to x-rays created by striking the target with 6 MeV electrons.) Moreover, because the intensity distribution of x-rays emitted from the target varies according to the x-ray energy, a different flattening filter must be used at each energy level. Generally speaking, higher the x-ray energy requires a thicker filter, i.e., a cone with a larger height. The need to use a different filter with each x-ray energy is an impediment to varying the x-ray energy of the beam in real time, and hence its penetration ability, during the course of treatment.
Likewise, known radiation therapy systems are capable of delivering particle beams at a plurality of energy levels, such that scattering foils, like x-ray flattening filters, must be carefully designed for a specific energy and a specific field size.